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Karl Jansen

Publications and source records attributed to Karl Jansen.

At least 289 records · Page 16Linked to original sources
arXiv2009

Scattering from finite size methods in lattice QCD

Using two flavors of maximally twisted mass fermions, we calculate the S-wave pion-pion scattering length in the isospin I=2 channel and the P-wave pion-pion scattering phase in the isospin I=1 channel. In the former channel, the lattice calculations are performed at pion masses ranging from 270 MeV to 485 MeV. We use chiral perturbation theory at next-to-leading order to extrapolate our results. At the physical pion mass, we find m_pi a_pipi(I=2)=-0.04385(28)(38) for the scattering length. In the latter channel, the calculation is currently performed at a single pion mass of 391 MeV. Making use of finite size methods, we evaluate the scattering phase in both the center of mass frame and the moving frame. The effective range formula is employed to fit our results, from which the rho resonance mass and decay width are evaluated.

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arXiv2009

Chirally rotated Schroedinger functional: non-perturbative tuning in the quenched approximation

The use of chirally rotated boundary conditions provides a formulation of the Schroedinger functional that is compatible with automatic O(a)-improvement of Wilson fermions in the bulk. The elimination of bulk O(a) terms requires the non-perturbative tuning of the critical mass and one additional boundary counterterm. We present the results of such a tuning in the quenched approximation at three values of the renormalised gauge coupling and for a range of lattice spacings.

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arXiv2009

Continuum-limit scaling of overlap fermions as valence quarks

We present the results of a mixed action approach, employing dynamical twisted mass fermions in the sea sector and overlap valence fermions, with the aim of testing the continuum limit scaling behaviour of physical quantities, taking the pion decay constant as an example. To render the computations practical, we impose for this purpose a fixed finite volume with lattice size $L\approx1.3$ fm. We also briefly review the techniques we have used to deal with overlap fermions.

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arXiv2008

Lattice QCD: a critical status report

The substantial progress that has been achieved in lattice QCD in the last years is pointed out. I compare the simulation cost and systematic effects of several lattice QCD formulations and discuss a number of topics such as lattice spacing scaling, applications of chiral perturbation theory, non-perturbative renormalization and finite volume effects. Additionally, the importance of demonstrating universality is emphasized.

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arXiv2008

Higgs mass bounds from a chirally invariant lattice Higgs-Yukawa model with overlap fermions

We study the parameter dependence of the Higgs mass in a chirally invariant lattice Higgs-Yukawa model emulating the same Higgs-fermion coupling structure as in the Higgs sector of the electroweak Standard Model. Eventually, the aim is to establish upper and lower Higgs mass bounds. Here we present our preliminary results on the lower Higgs mass bound at several selected values for the cutoff and give a brief outlook towards the upper Higgs mass bound.

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arXiv2008

Status of ETMC simulations with Nf=2+1+1 twisted mass fermions

We present the status of runs performed in the twisted mass formalism with $N_{\rm f}=2+1+1$ flavours of dynamical fermions: a degenerate light doublet and a mass split heavy doublet. The procedure for tuning to maximal twist will be described as well as the current status of the runs using both thin and stout links. Preliminary results for a few observables obtained on ensembles at maximal twist will be given. Finally, a reweighting procedure to tune to maximal twist will be described.

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arXiv2008

The static-light meson spectrum from twisted mass lattice QCD

We compute the static-light meson spectrum with N_f = 2 flavours of sea quarks using Wilson twisted mass lattice QCD. We consider five different values for the light quark mass corresponding to 300 MeV < m_PS < 600 MeV and we present results for angular momentum j = 1/2, j = 3/2 and j = 5/2 and for parity P = + and P = -. We extrapolate our results to physical quark masses and make predictions regarding the spectrum of B and B_s mesons.

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arXiv2008

Static-light meson masses from twisted mass lattice QCD

We compute the static-light meson spectrum using two-flavor Wilson twisted mass lattice QCD. We have considered five different values for the light quark mass corresponding to 300 MeV < m_PS < 600 MeV. We have extrapolated our results, to make predictions regarding the spectrum of B and B_s mesons.

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arXiv2007

The two flavour Schwinger model: scaling of the scalar condensate

We investigate the continuum limit scaling of the scalar condensate in the $N_f=2$ Schwinger model on the lattice. We employ maximally twisted mass Wilson fermions and overlap fermions. We compute the scalar condensate by taking the trace of the propagator (direct method) and by utilizing the integrated Ward-Takahashi identity. While the scalar condensate comes out consistent using these two methods for a given kind of lattice fermions, we find --quite surprisingly-- large discrepancies for the scalar condensate between twisted mass and overlap fermions. These discrepancies are only resolved when using the point split current for twisted mass fermions.

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arXiv2007

Moments of meson distribution functions with dynamical twisted mass fermions

We present our preliminary results on the lowest moment of quark distribution functions of the pion using two flavor dynamical simulations with Wilson twisted mass fermions at maximal twist. The calculation is done in a range of pion masses from 300 to 500 MeV. A stochastic source method is used to reduce inversions in calculating propagators. Finite volume effects at the lowest quark mass are examined by using two different lattice volumes. Our results show that we achieve statistical errors of only a few percent. We plan to compute renormalization constants non-perturbatively and extend the calculation to two more lattice spacings and to the nucleons.

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arXiv2007

The phase structure of a chirally invariant lattice Higgs-Yukawa model

We consider a chirally invariant lattice Higgs-Yukawa model based on the Neuberger overlap operator. As a first step towards the eventual determination of Higgs mass bounds we present the phase structure of the model analytically in the large Nf-limit in the physically interesting region of the Yukawa coupling constant. We confront the analytically obtained phase diagram with corresponding HMC-simulations and find an excellent agreement at large values of Nf. In the opposite case the large Nf computation still gives a good qualitative description of the phase diagram. We also present first and very preliminary results on the Higgs upper bound at one selected cut-off of the theory.

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arXiv2007

On the phase structure of a chiral invariant Higgs-Yukawa model

In the past the construction of Higgs-Yukawa models on the lattice was blocked by the lack of a consistent definition of a chiral invariant Yukawa coupling term. Here, we consider a chiral invariant Higgs-Yukawa model based on the overlap operator, realized by the Neuberger-Dirac operator. As a first step towards a numerical examination of this model we study its phase diagram by means of an analytic 1/N-expansion, which is possible for small and for large values of the Yukawa coupling constant. In the case of strong Yukawa couplings the model effectively becomes an O(4)-symmetric non-linear sigma-model.

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arXiv2006

Status Report on ILDG activities

A status report about the International Lattice Data Grid (ILDG) is given. Different countries participating in the ILDG have created regional lattice data grid solutions that are implemented, working and used. The remaining task and the focus of present activities is the development of the interoperability of these regional grids. A first, successful step in this direction is a metadata catalogue service which is already working interoperable.

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arXiv2006

First results with two light flavours of quarks with maximally twisted mass

We report on first results of an ongoing effort to simulate lattice QCD with two degenerate flavours of quarks by means of the twisted mass formulation tuned to maximal twist. By utilising recent improvements of the HMC algorithm, pseudo-scalar masses well below 300 MeV are simulated on volumes with spatial size larger than 2 fm at values of the lattice spacing similar or smaller than 0.1 fm. We present first evidence that scaling violations in the pseudo-scalar decay constant are small and well compatible with O(a) improvement. Additionally, exploratory results for the case of N_f=2+1+1 flavours are discussed.

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arXiv2005

Two--dimensional lattice Gross--Neveu model with Wilson twisted mass fermions

We study the two-dimensional lattice Gross--Neveu model with Wilson twisted mass fermions in order to explore the phase structure in this setup. In particular, we investigate the behaviour of the phase transitions found earlier with standard Wilson fermions as a function of the twisted mass parameter $μ$. We find that qualitatively the dependence of the phase transitions on $μ$ is very similar to the case of lattice QCD.

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arXiv2005

Testing Fermion Actions: Scaling in the Schwinger Model

We test the scaling behaviour of Wilson, hypercube, maximally twisted mass and overlap fermion actions in dynamical simulations of the 2-dimensional massive Schwinger model. We also present possibilities to simulate overlap fermions dynamically by replacing the exact overlap operator by an approximate version. This approximation is used either as only the guidance Hamiltonian, keeping the exact overlap operator as the accept/reject Hamiltonian or for both, the guidance and accept/reject Hamiltonian in the Hybrid Monte Carlo algorithm. In the latter case we test whether the approximation error can be corrected for by computing the determinant ratio of the exact and the approximate overlap operators stochastically.

hep-lat↗
arXiv2004

Scaling test for Wilson twisted mass QCD

We present a first scaling test of twisted mass QCD with pure Wilson quarks for a twisting angle of pi/2. We have computed the vector meson mass and the pseudoscalar decay constant for different values of beta at fixed value of r_0 m_PS. The results obtained in the quenched approximation are compared with data for pure Wilson and non-perturbatively O(a) improved Wilson computations. We show that our results from Wilson twisted mass QCD show clearly reduced lattice spacing errors, consistent with O(a) improvement and without the need of any improvement terms added. These results thus provide numerical evidence of the prediction in ref. [1].

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